The present invention relates to a method for controlling an internal combustion engine associated with an exhaust gas recirculation system (hereafter referred to as "EGR" as an exhaust emission control system. More specifically, the invention relates to a method for deriving an accurate value of an intake air mass flow rate (cylinder air mass flow rate) actually introduced into an engine cylinder during active state of the EGR system, which is necessary for engine control, and a method for precisely determining a fuel injection amount with taking delay of transportion of fuel through an intake manifold.
For the purpose of exhaust emission control and fuel economy, modern automotive internal combustion engines employ electronic control systems which maintain air/fuel mixture ratio at a target value throughout the engine operation. In order to control air/fuel ratio with high precision, a cylinder air mass flow rate to be actually introduced into an engine cylinder is precisely detected and necessary fuel flow amount is determined on the basis of the intake air mass flow rate value and the target air/fuel ratio value.
The co-pending U.S. patent application Ser. No. 07/640,598, filed by the same applicant to the present invention on Jan. 10, 1991, pending, and commonly assigned to the assignee of the present invention, discloses a method for determining the intake air mass flow rate to be introduced into the engine cylinder by measuring an intake air mass flow rate flowing into an intake manifold by means of an air flow meter and deriving an internal pressure in the intake manifold on the basis of the measured intake air mass flow rate and further deriving the actually introduced intake air mass flow rate on the basis of the intake air pressure in the intake manifold and an engine speed. In this method, taking the intake air pressure and the engine speed as parameters, the cylinder air mass flow rate corresponding to both parameters is preliminarily measured through experiments. The experimentally measured cylinder air mass flow rate values are mapped in a form of a table. Therefore, the cylinder air mass flow rate is determined by looking up the table in terms of the intake air pressure and the engine speed.
Furthermore, in Japanese Unexamined Patent Publication (Kokai) No. 58-8238, for Japanese Patent Application filed by Toyota Motor Company, Limited on Jul. 6, 1981, discloses a method for determining a fuel injection amount with taking an intake manifold wetting fuel ratio which adhere on the inner periphery of the intake manifold and delivery ratio of the wetting fuel into the engine cylinder, with taking the intake manifold wetting fuel ratio and the wetting fuel delivery ratio as parameters representative of the engine operating condition.
Here, in the recent years, for the purpose of environment protection, emission restriction value for nitrogen oxides (NO.sub.x) is set by a strict emission control regulation. In order to satisfy such restriction value, most of the internal combustion engines employs EGR systems. As is well known, the ERG system recirculates part of the exhaust gas exhausted from an exhaust port of the engine into the air intake manifold trough an EGR valve to introduce into the engine cylinder together with the fresh intake air. By effecting recirculation of the exhaust gas through the EGR system, maximum temperature of combustion in a combustion chamber is lowered to reduce generation amount of NO.sub.x. On the other hand, the charge efficiency of the intake air into the engine cylinder is variable between active state and inactive state of EGR even when the intake air pressure, the engine speed and the intake air temperature are maintained constant.
Accordingly, when the cylinder air mass flow rate value derived at inactive state of the EGR is applied for derivation of the fuel injection amount in the active state of EGR, the actual air/fuel ratio cannot be controlled to the target air/fuel ratio and can become over rich or over lean.
On the other hand, since mobility of the gas (air and exhaust gas) in the intake manifold is variable depending upon active and inactive states of EGR, a transfer characteristics of the injected fuel into the engine cylinder is variable even at the constant engine operating condition. Accordingly, when the fuel transfer characteristics determined at the inactive state of the EGR is applied for derivation of the fuel injection amount at the active state of EGR, a problem can be encountered to cause an error in air/fuel ratio control.
Despite of the fact as set forth above, since the applicants' co-pending application and the Japanese publication are not directed to derivation of the cylinder air mass flow rate or the fuel injection amount at the active state of EGR, the foregoing problems can be encountered.